Bottom Line:
A suitable LAB/PNSB ratio and initial cell concentration were found to be 1/12 (w/w) and 0.15 g/L, respectively.The ratio of the strains TISTR 895/KKU-PS5 and their initial cell concentrations affected the rate of lactic acid production and its consumption.A suitable LAB/PNSB ratio and initial cell concentration could balance the lactic acid production rate and its consumption in order to avoid lactic acid accumulation in the fermentation system.

Background: Bioaugmentation or an addition of the desired microorganisms or specialized microbial strains into the anaerobic digesters can enhance the performance of microbial community in the hydrogen production process. Most of the studies focused on a bioaugmentation of native microorganisms capable of producing hydrogen with the dark-fermentative hydrogen producers while information on bioaugmentation of purple non-sulfur photosynthetic bacteria (PNSB) with lactic acid-producing bacteria (LAB) is still limited. In our study, bioaugmentation of Rhodobacter sphaeroides KKU-PS5 with Lactobacillus delbrueckii ssp. bulgaricus TISTR 895 was conducted as a method to produce hydrogen. Unfortunately, even though well-characterized microorganisms were used in the fermentation system, a cultivation of two different organisms in the same bioreactor was still difficult because of the differences in their metabolic types, optimal conditions, and nutritional requirements. Therefore, evaluation of the physical and chemical factors affecting hydrogen production of PNSB augmented with LAB was conducted using a full factorial design followed by response surface methodology (RSM) with central composite design (CCD).

Results: A suitable LAB/PNSB ratio and initial cell concentration were found to be 1/12 (w/w) and 0.15 g/L, respectively. The optimal initial pH, light intensity, and Mo concentration obtained from RSM with CCD were 7.92, 8.37 klux and 0.44 mg/L, respectively. Under these optimal conditions, a cumulative hydrogen production of 3396 ± 66 mL H2/L, a hydrogen production rate (HPR) of 9.1 ± 0.2 mL H2/L h, and a hydrogen yield (HY) of 9.65 ± 0.23 mol H2/mol glucose were obtained. KKU-PS5 augmented with TISTR 895 produced hydrogen from glucose at a relatively high HY, 9.65 ± 0.23 mol H2/mol glucose, i.e., 80 % of the theoretical yield.

Conclusions: The ratio of the strains TISTR 895/KKU-PS5 and their initial cell concentrations affected the rate of lactic acid production and its consumption. A suitable LAB/PNSB ratio and initial cell concentration could balance the lactic acid production rate and its consumption in order to avoid lactic acid accumulation in the fermentation system. Through use of appropriate environmental conditions for bioaugmentation of PNSB with LAB, a hydrogen production could be enhanced.

Mentions:
Based on our findings, we further investigated the effect of PNSB concentration on hydrogen production at a fixed initial LAB concentration of 0.03 g/L and LAB/PNSB ratios of 1/2 and 1/7. Figure 1 shows the time-course profiles of hydrogen production, cell, glucose, and metabolite (lactic acid, formic acid) concentrations of LAB fermentation at an initial LAB concentration of 0.03 g/L (Fig. 1a); PNSB augmented with LAB at a LAB/PNSB ratio of 1/2 with a LAB concentration of 0.033 g/L and a PNSB concentration of 0.067 g/L (Fig. 1b) (Condition B2); and PNSB augmented with LAB at a LAB/PNSB ratio of 1/7 with a LAB concentration of 0.031 g/L and a PNSB concentration of 0.219 g/L (Fig. 1c) (Condition C5). Approximately 1.8 g/L of lactic acid was produced by LAB (Fig. 1a) with 2.1 g/L of glucose utilized, or 86 % of the substrate was consumed for lactic acid formation by LAB (Fig. 1a). LAB concentration was increased from 0.03 to approximately 0.25 g/L. The results suggested that glucose was consumed by LAB to produce lactic acid and to maintain the cells without hydrogen production. Augmentation of LAB in the system at a LAB/PNSB ratio of 1/2 produced hydrogen at 721 ± 65 mL H2/L (Fig. 1b). Under this condition, lactic acid accumulated in the fermentation broth to about 0.66 ± 0.06 g/L, indicating that the amount of PNSB (0.067 g/L) added in the system was not high enough to balance the rate of lactic acid production and its consumption. When the concentration of PNSB was increased to 0.219 g/L at a LAB/PNSB ratio of 1/7 (Fig. 1c), hydrogen production increased to 1359 ± 129 mL H2/L, which was approximately two times higher than when the LAB/PNSB ratio was 1/2. The concentration of residual lactic acid in the fermentation broth was low. In addition, not only lactic acid was used as substrate to produce hydrogen by PNSB, but also glucose. Increasing the PNSB level at this LAB/PNSB ratio produced a higher lactic acid consumption rate, which in turn reduced lactic acid accumulation. Therefore, at a fixed LAB concentration, the cell concentration of PNSB could become a limiting factor for hydrogen production in a bioaugmentation system.Fig. 1

Mentions:
Based on our findings, we further investigated the effect of PNSB concentration on hydrogen production at a fixed initial LAB concentration of 0.03 g/L and LAB/PNSB ratios of 1/2 and 1/7. Figure 1 shows the time-course profiles of hydrogen production, cell, glucose, and metabolite (lactic acid, formic acid) concentrations of LAB fermentation at an initial LAB concentration of 0.03 g/L (Fig. 1a); PNSB augmented with LAB at a LAB/PNSB ratio of 1/2 with a LAB concentration of 0.033 g/L and a PNSB concentration of 0.067 g/L (Fig. 1b) (Condition B2); and PNSB augmented with LAB at a LAB/PNSB ratio of 1/7 with a LAB concentration of 0.031 g/L and a PNSB concentration of 0.219 g/L (Fig. 1c) (Condition C5). Approximately 1.8 g/L of lactic acid was produced by LAB (Fig. 1a) with 2.1 g/L of glucose utilized, or 86 % of the substrate was consumed for lactic acid formation by LAB (Fig. 1a). LAB concentration was increased from 0.03 to approximately 0.25 g/L. The results suggested that glucose was consumed by LAB to produce lactic acid and to maintain the cells without hydrogen production. Augmentation of LAB in the system at a LAB/PNSB ratio of 1/2 produced hydrogen at 721 ± 65 mL H2/L (Fig. 1b). Under this condition, lactic acid accumulated in the fermentation broth to about 0.66 ± 0.06 g/L, indicating that the amount of PNSB (0.067 g/L) added in the system was not high enough to balance the rate of lactic acid production and its consumption. When the concentration of PNSB was increased to 0.219 g/L at a LAB/PNSB ratio of 1/7 (Fig. 1c), hydrogen production increased to 1359 ± 129 mL H2/L, which was approximately two times higher than when the LAB/PNSB ratio was 1/2. The concentration of residual lactic acid in the fermentation broth was low. In addition, not only lactic acid was used as substrate to produce hydrogen by PNSB, but also glucose. Increasing the PNSB level at this LAB/PNSB ratio produced a higher lactic acid consumption rate, which in turn reduced lactic acid accumulation. Therefore, at a fixed LAB concentration, the cell concentration of PNSB could become a limiting factor for hydrogen production in a bioaugmentation system.Fig. 1

Bottom Line:
A suitable LAB/PNSB ratio and initial cell concentration were found to be 1/12 (w/w) and 0.15 g/L, respectively.The ratio of the strains TISTR 895/KKU-PS5 and their initial cell concentrations affected the rate of lactic acid production and its consumption.A suitable LAB/PNSB ratio and initial cell concentration could balance the lactic acid production rate and its consumption in order to avoid lactic acid accumulation in the fermentation system.

Background: Bioaugmentation or an addition of the desired microorganisms or specialized microbial strains into the anaerobic digesters can enhance the performance of microbial community in the hydrogen production process. Most of the studies focused on a bioaugmentation of native microorganisms capable of producing hydrogen with the dark-fermentative hydrogen producers while information on bioaugmentation of purple non-sulfur photosynthetic bacteria (PNSB) with lactic acid-producing bacteria (LAB) is still limited. In our study, bioaugmentation of Rhodobacter sphaeroides KKU-PS5 with Lactobacillus delbrueckii ssp. bulgaricus TISTR 895 was conducted as a method to produce hydrogen. Unfortunately, even though well-characterized microorganisms were used in the fermentation system, a cultivation of two different organisms in the same bioreactor was still difficult because of the differences in their metabolic types, optimal conditions, and nutritional requirements. Therefore, evaluation of the physical and chemical factors affecting hydrogen production of PNSB augmented with LAB was conducted using a full factorial design followed by response surface methodology (RSM) with central composite design (CCD).

Results: A suitable LAB/PNSB ratio and initial cell concentration were found to be 1/12 (w/w) and 0.15 g/L, respectively. The optimal initial pH, light intensity, and Mo concentration obtained from RSM with CCD were 7.92, 8.37 klux and 0.44 mg/L, respectively. Under these optimal conditions, a cumulative hydrogen production of 3396 ± 66 mL H2/L, a hydrogen production rate (HPR) of 9.1 ± 0.2 mL H2/L h, and a hydrogen yield (HY) of 9.65 ± 0.23 mol H2/mol glucose were obtained. KKU-PS5 augmented with TISTR 895 produced hydrogen from glucose at a relatively high HY, 9.65 ± 0.23 mol H2/mol glucose, i.e., 80 % of the theoretical yield.

Conclusions: The ratio of the strains TISTR 895/KKU-PS5 and their initial cell concentrations affected the rate of lactic acid production and its consumption. A suitable LAB/PNSB ratio and initial cell concentration could balance the lactic acid production rate and its consumption in order to avoid lactic acid accumulation in the fermentation system. Through use of appropriate environmental conditions for bioaugmentation of PNSB with LAB, a hydrogen production could be enhanced.